The present invention relates to a new and improved construction of an optical smoke detector of the type comprising a radiation source which transmits radiation throughout a predetermined spatial region, and at least one radiation receiver to which there is delivered the radiation which is scattered by particles located in the radiation region.
With smoke detectors of this general character it is possible to select the radiation in the visible, infrared or ultraviolet wavelength range, depending upon the nature of the smoke particles to be detected. With such smoke detectors, as utilized for instance in the fire alarm art, the radiation receiver is not directly impinged or irradiated, rather arranged externally of the radiation range or region such that it only then receives radiation when radiation-scattering particles enter the radiation path and cause scattering of the radiation. Typical of such type optical smoke detectors are those disclosed in the commonly assigned U.S. Pat. No. 3,316,410, granted Apr. 25, 1976 and U.S. Pat. No. 3,760,395, granted Sept. 18, 1973, to which reference may be readily had, and the disclosure of which is incorporated herein by reference. As soon as the scattered radiation intensity, received by the radiation receiver, has attained a certain value, then a signal is delivered by a suitable evaluation circuit, for instance in the manner taught, by way of example, in Swiss Pat. No. 417,405, or the Japanese petty patent publications Sho No. 47-21577, 47-21578, and 48-2687 and the Japanese patent publication Sho No. 47-32797, the disclosure of which is also incorporated herein by reference.
Heretofore known smoke detectors of this general character transmit the radiation by means of an optical system into a measuring chamber. The radiation receiver is arranged transversely with respect to the radiation direction such that it preferably can receive radiation which is scattered through an angle of 90.degree.. The efficiency of such arrangement is, however, relatively poor, since the irradiation or impingement of the radiation receiver is extremely small when there prevails low smoke density in the measuring chamber. Therefore, such smoke detectors are associated with the drawback that when used as fire alarms they do not react early enough to the first traces of smoke originating when a fire breaks out.
It has already been attempted to make use of the fact that for most types of particles which are to be detected the forward radiation scattering--during which the receiving direction forms an acute angle with the radiation direction--is greater than the sideward scattering or rearward scattering. Hence, the radiation receiver is dispositioned such that it is just still located externally of the radiation bundle. However, the sensitivity increase which can be obtained with such smoke detectors falls within narrow limits, since even here there is only used a very small part of the scattered radiation. Additionally, the radiation must be focused or bundled extremely well in order that the radiation receiver is not impinged by direct peripheral or marginal radiation, rendering such equipment quite expensive and difficult to adjust.